Tunable narrow-band filter for imaging polarimetry

Transcription

1 **FULL TITLE** ASP Conference Series, Vol. **VOLUME**, **YEAR OF PUBLICATION** **NAMES OF EDITORS** Tunable narrow-band filter for imaging polarimetry A. Feller 1, A. Boller 1, J.O. Stenflo 1,2 1 Institute of Astronomy, ETH Zentrum, 8092 Zurich 2 Faculty of Mathematics & Science, University of Zurich Abstract. A fully tunable narrow-band filter system to be used in combination with ZIMPOL for monochromatic imaging vector polarimetry is being developed. It may be used over the whole visible spectrum, from the UV at about 395 nm, to the red at about 660 nm, with a band width of må. The main components are two lithium niobate Fabry-Perot etalons made of Y-cut crystals, which means that the channel spectra and tuning parameters are different for the two orthogonal states of polarization (ordinary and extraordinary rays). This allows the finesse to be dramatically enhanced by a double-pass configuration, a possibility that is not available to other Fabry-Perot systems. Tuning can be achieved in three ways: temperature tuning to center the pass band within the selected fine tuning range, voltage tuning for rapid fine tuning, and tilt tuning as an additional, though normally not used, possibility. The filter system will allow us to explore the spatial structuring of the polarization signatures in the Second Solar Spectrum, including vector mapping of the Hanle and Zeeman effects in any spectral line between the UV and red part of the spectrum. 1. Introduction During the last decade our institute has developed a powerful CCD-based imaging polarimeter system ZIMPOL (Zurich Imaging Polarimeter) (cf. Povel 1995; Gandorfer et al. 2004). Until now ZIMPOL has mainly been used in combination with a spectrograph (cf. Stenflo 2004). The narrow-band filter system will allow us to combine high-precision polarimetry with monochromatic imaging, thus providing a new scientific dimension to our work with ZIMPOL. An exploratory attempt in this direction with the UBF at Sac Peak has given a first glimpse of the type of data that may be obtained (Stenflo et al. 2002). 2. Optical setups Basically two types of choices can be made: collimated or telecentric setups. The collimated setup has the main advantage that the FPE (Fabry-Perot etalon) passband is unbroadened and thus narrower as compared with the telecentric setup. The disadvantage is that the central wavelength of the passband varies somewhat across the field of view. In contrast, the telecentric setup produces a constant but broadened passband across the field of view. 1

2 2 Feller, Boller, and Stenflo There is another important aspect to the design of FPE optics. Following Darvann & Owner-Petersen (1994) λ D FPE α FOV D tel λ. (1) No matter what type of setup one is choosing, one has to make a trade-off between field of view α FOV and spectral resolution λ/ λ. For a collimated setup, λ corresponds to the passband shift across the field of view. For a telecentric setup λ has to be interpreted as the spectral broadening. D FPE and D tel are the diameters of the illuminated apertures of the FPE and the telescope respectively. We will use both types of setups (cf. Fig. 1 and Table 1) and have chosen the focal lengths such that λ is less than the FWHM of a single etalon. The collimated setup makes use of the maximum field of view available at the IRSOL telescope and aims at high polarimetric sensitivity. It will also allow us to experiment with a double pass configuration (Netterfield et al. 1997). Between the two passes we rotate the plane of linear polarization by 90 with a retarder. Since our Y-cut LiNbO 3 crystals are birefringent, this has the effect that a single etalon will provide a similar effective finesse as two etalons in series with different cavities. With both our etalons used in double pass the transmission becomes equivalent to that of a 4-etalon system, which dramatically increases the effective finesse. The telecentric setup aims at high spatial resolution and portability to allow the use at foreign facilities like the NSST at La Palma. Table 1. Optical setups at IRSOL collimated telecentric camera illumination F/44 F/270 field of view spatial resolution 1 sampling limited: diffraction limited: to 0.4 FPE passband field-dependent blueshift: 22 må broadening: 1 må used FPE aperture between 65% and 100% 36% imaging optics two spherical mirrors 2 achromatic lenses unfolded setup length 9.3 m 1.4 m 1 without seeing 3. Tuning and Calibration The transmission profile of an etalon can be shifted by varying the optical thickness of the cavity. With our LiNbO 3 etalons this can be achieved in three ways. Tilting broadens the transmission profile and is mainly used for ghost suppression and for double pass mode. Temperature tuning is slow and is used to set the FPE at an optimum home position within a spectral region of interest.

3 Tunable narrow-band filter for imaging polarimetry 3 telescope focal plane pupil mirror 2 f 2 = 1700 mm double pass mirror 1 f 1 = 2160 mm FPE FPE quarter-wave plate camera focal plane FPE telescope focal plane pupil camera focal plane lens 1 f 1 = 97 mm lens 2 f 2 = 470 mm Figure 1. Schematic drawing of the two possible optical setups (not to scale). Top: Collimated setup. This setup is designed for low spatial but high spectral resolution. It also permits to experiment with a double-pass configuration. Bottom: Telecentric setup with an image scale near the diffraction limit of an F/50 solar telescope. Its compact design aims at portability. Tuning by voltage is much faster (about 3 FWHM/s). It is used for line scans or to switch between several discrete spectral positions, which is required for differential Zeeman and Hanle diagnostics. All three tuning parameters, tilt angle, temperature and voltage, can be controlled manually or via computer. Under computer control one has the possibility to access these parameters within the framework of the ZIMPOl II software. Thus one can script different measuring sequences and synchronize the individual etalons with the polarimeter. The best model of the refractive indices of LiNbO 3 known to us (Schlarb & Betzler 1993) is not good enough for a prediction of the transmission peaks over the whole spectral working range of the FPE with the required accuracy of less than a FWHM. We therefore calibrate the etalons for each prefilter separately. With the help of the spectrograph available at IRSOL we record the transmission spectrum for different temperatures T and voltages V. From these measurements we can derive λ i (T, V ) of each transmission peak i within the prefilter range with sufficient accuracy.

4 4 Feller, Boller, and Stenflo Table 2. Basic characteristics of the individual etalons cavity material LiNbO 3, Y-cut spectral range 390 to 650 nm cavity thickness ± /0.017 resp. 985 ± /0.006 µm effective finesse 2 20 free spectral range Å resp Å 1 First error value: rms of the higher order thickness fluctuations, after subtraction of the linear and quadratic terms. Second error value: peak to valley. 2 Combination of reflective and defect finesses (Atherton, Reay, & Ring 1981). 4. Performance The performance of an imaging polarimeter is defined by three criteria: spatial and spectral resolution as well as polarimetric accuracy. Here we summarize the main effects and their influence on the performance of our instrument. In our collimated setup, spatial resolution is not a major concern. We have verified that the influence of the thickness fluctuations across the etalon apertures on the point spread function are negligible in our case. The thickness fluctuations however induce local passband shifts of order several tenths of a FWHM. As a consequence the light throughput is diminished and the passband of the FPE broadened, which affects polarimetric accuracy and spectral resolution. In the telecentric setup we sample the Airy radius with about two pixels. Pupil apodization, which produces a wavelength-dependent point spread function (Beckers 1998; Von der Lühe & Kentischer 2000), has therefore a significant influence on our spatial resolution. The optical thickness fluctuations manifest themselves as local variations of the spectral transmission profile across the field of view. When combining several etalons and prefilters, the resulting spectral profile is never perfectly limited to one transmission peak, but there are different sources of parasitic light leaking through. The parasitic light can have a polarized contribution P par and an unpolarized contribution I par, and its effect on the measured degree of polarization is twofold: ( P I ) meas = P I I par /I + P par I par I/I par. (2) The first term describes the dilution of the true signal P/I. The second term shows that the measured signal (P/I) meas can be contaminated from a neighboring polarized spectral line. The sources of parasitic light have different origins: the transmission ghosts directly result from the multiplication of the transmission profiles, whereas the reflection ghosts come from reflections between the etalons. As shown by Darvann & Owner-Petersen (1994) one important measure in reducing the amount of parasitic light from transmission ghosts is a careful choice of the cavity-thickness ratios. Furthermore the choice of the prefilter parameters (number of cavities,

5 Tunable narrow-band filter for imaging polarimetry 5 FWHM) is vital for the effective isolation of one transmission peak out of the channel spectrum. To suppress the reflection ghosts between the etalons we are applying two different methods. The first method is to tilt one etalon relative to the other one. In the collimated setup the field-dependent wavelength shifts of both etalons are then no longer matching each other, which limits the tilt angle to small values. The second method is to insert the prefilter between the etalons, as suggested by Tritschler et al. (2002). This method however requires prefilters with a large aperture and good optical quality, especially for the collimated setup transmission wavelength [nm] Figure 2. Numerical model of the FPE transmission profile in the collimated setup. As an example the region around the Sr I line has been chosen. Dotted line: transmission profile of a 3-cavity prefilter. Thick solid line: main transmission peak. Weak solid line: transmission ghosts. The reflection ghosts (dashed line) contain 280% of the energy of the main transmission peak. When the prefilter is located between the etalons (dash-dotted line) this amount is reduced to 5%. A numerical model of the FPE transmission (Fig. 2), based on Darvann & Owner-Petersen (1994) and Martínez Pillet et al. (2004), helps us to select the best combination of the available etalon cavities (ordinary or extraordinary axis) and the prefilter parameters for a given spectral region of interest. Acknowledgments. Support for this work has been obtained through grants Nos and from the Swiss Nationalfonds and grant No from ETH Zurich.

6 6 Feller, Boller, and Stenflo 80 blue wing line center red wing I 20 0 arcsec L/I V/I arcsec Figure 3. First light observation with the FPE in combination with ZIM- POL at IRSOL. Within the passband of a prefilter with 0.5 Å FWHM, centered on Hα, the FPE was tuned to the blue wing, red wing, and line center, respectively. For each spectral position a series of Stokes image sets were recorded. The total integration time per series is about 50s. Since the linear polarization is weak, Q/I and U/I have been combined to an amplitude of linear polarization L/I = [(Q/I) 2 +(U/I) 2 ] 1/2. The full greyscale corresponds to 40% of the mean value for I, and to 1.5% for L/I and V/I. Prior to averaging, the individual images have been numerically corrected for rotation and shifts, which produces artefacts at some of the image edges. References Atherton, P. D., Reay, N. K., & Ring, J. 1981, Opt. Eng., 20, 806 Beckers, J. M. 1998, A&AS, 129, 191 Darvann T., & Owner-Petersen, M. 1994, LEST technical report 57 Gandorfer, A. M. et al. 2004, A&A, 422, 703 Martínez Pillet, V. et al. 2004, Proc. of SPIE, 5487, 1152 Netterfield, R. P., Freund, C. H., Seckold, J. A., & Walsh, C. J. 1997, Appl.Optics, 36, 4556 Povel, H. 1995, Optical Engineering, 34, 1870 Schlarb, U. & Betzler, K. 1993, Phys. Rev. B, 48, Stenflo, J.O. 2004, Rev. Mod. Astron., 17, 269 Stenflo, J.O., Gandorfer, A., Holzreuter, R., Gisler, D., Keller, C.U., Bianda, M. 2002, A&A, 389, 314 Tritschler, A., Schmidt, W., Langhans, K., & Kentischer T. 2002, Solar Phys., 211, 17 Von der Lühe, O. & Kentischer Th. J. 2000, A&A, 146, 499